The present invention relates to an image recording apparatus, and particularly to an image recording apparatus in which an optical fiber is used in a recording laser head to thereby improve the utility efficiency of energy when images, characters, etc. are recorded on a recording medium.
FIGS. 6(a) and 6(b) show a fundamental structure of an optical fiber. An optical fiber usually has a layer 2 called a clad portion and formed around a core portion (light transmitting portion) 1 which is circular in section. Optical fibers are roughly classified into two types, that is, a step-index type and a graded-index type. The step-index type optical fiber is designed so that the refractive index is stepwise changed in the interface between the core portion and the clad portion of the optical fiber. The refractive index of the core portion is made constant to be n1, while the refractive index of the clad portion is made constant to be n2 (n1&gt;n2). On the other hand, the graded-index type optical fiber is designed so that the refractive index of the core portion is changed gradually from the center portion toward the outer circumference. Specifically, the refractive index of the core portion takes a maximum at the center portion, which it decreases as the portion goes toward the interface with the clad portion.
FIG. 7(a) shows the refractive index of the core portion and that of the clad portion in the step-index type optical fiber, and FIG. 8(a) shows the refractive index of the core portion and that of the clad portion in the graded-index type optical fiber. In addition, FIG. 7(b) and FIG. 8(b) show intensity profiles of light emitted from the step-index type optical fiber and the graded-index type optical fiber, respectively, when the optical fiber is connected to a laser light source through a lens or the like. In the FIGS. 7(b) and 8(b), dashed lines indicate the refractive index of the respective core portions.
FIGS. 9(a) and 9(b) three-dimensionally show the intensity profiles of light emitted from the step-index type and graded-index type optical fibers respectively. That is, FIG. 9(a) shows a top-hat shaped profile of laser light emitted from the step-index type optical fiber. FIG. 9(b) shows a substantially Gaussian profile of laser light emitted from the graded-index type optical fiber.
In each of the laser light emission profiles of the top-hat shape and the substantially Gaussian shape shown in FIGS. 9(a) and 9(b), there is a problem that the energy of laser light cannot be utilized effectively. That is, though the energy could be utilized most effectively in a laser printer if the dot shape could be made square as much as possible, there is a problem that though recording can be attained only in the vicinity of the central portion of the light emission profile, it cannot be performed in the circumferential portion, because the dot shape is not completely square, as shown in FIG. 7(b), FIG. 8(b), and FIGS. 9(a) and 9(b) showing the dot shape in three dimensions, and the insufficiency of light intensity.